The present invention relates to a photosensitive body for electrophotography which may be used in, for example an electrophotographic copying apparatus or a laser printer.
As a result of the significant proliferation of a variety of word processors and personal computers in recent years, the market demand for image printing apparatuses such as electrophotographic copying apparatuses and printers has sharply grown. Furthermore, as a result of the successful commercialization of a variety of photoconductors electrophotographic printers using such photoconductors have achieved remarkable progress. In particular, modern electrophotographic copying apparatuses primarily make use of a photosensitive body for embodying electrophotographic operations. Image quality, copying speed, power consumption, cost, etc. are mainly dependent on the physical performance characteristics of the employed photosensitive body constituted from photoconductive material. On the other hand, speaking of printers, laser printers using electrophotographic photosensitive body attract attentions of the concerned.
For example, an electrophotographic photosensitive body used in an electrophotographic apparatus is described below. FIG. 20 is a sectional view of a conventional photosensitive body. A reference numeral 14 designates a substrate constituted from conductive material and a reference numeral 15 indicates a photosensitive layer composed of photoconductive material which exhibits photoconductivity upon exposure to an irradiated light beam. Conventionally, a photosensitive layer is made from an inorganic material like Se, Se-As, Se-Te, a-Si, or Cds and so on, or from a polynuclear aromatic compound like anthracene, or from an organic material like phthalocyanine, or polyvinyl carbazole and so on. A photosensitive layer made from organic material is generally called "OPC". OPC is particularly used in electrophotographic copying apparatuses operating at slow and medium speeds because of its reduced harmfulness, lower cost, reduced hardness and reduced sensitivity when compared to the inorganic photosensitive layers available today. Some of the most recent organic photosensitive layers have a photosensitivity equivalent to that of an inorganic photosensitive layer. Due to this advantage, the organic photosensitive layers are used in some of the copying apparatuses operating at a fast speed.
Referring to FIGS. 21(a) and (b), organic photosensitive layers 25,35 respectively consist of both a charge generating layer (hereinbelow referred to as CGL) 16 and a charge transfer layer (hereinbelow referred to as CTL) 17. There are two types of layer structures including the layer structure 25 (referred to as a CTL/CGL/substrate structure) having a laminate of types of layer structures a substrate 14, a CGL 16 and a CTL 17 arranged in this order, and the other layer structure 35 (referred to as a CGL/CTL/substrate structure) 35 having a laminate of a substrate 14, a CTL 17 and a CGL 16 arranged in this order. Of these, the former CTL/CGL/substrate structure is widely used. This is because the CTL 17 has 20 to 30 .mu.m of thickness in contrast with the CGL 16 having 0.2 to 0.5 .mu.m of thickness, and thus, the CTL 17 is more resistant against wear than the CGL 16. More particularly, any electrophotographic copying apparatus executes a copying operation by following four sequential processes including (1) allowing an ozonizer to charge the surface of a photosensitive layer, (2) forming an image on the surface of the photosensitive layer by executing a light-exposure process and a development process with toner, (3) transferring the image onto a copying paper which is brought into contact with the surface of the photosensitive layer, and (4) scraping off residual toner from the surface of the photosensitive layer by applying a blade thereof. While executing this four-step processes, the blade comes into strong contact with the surface of the photosensitive layer, and thus, it severely affects the resistance of the photosensitive layer against wear. In other words, the blade adversely affects the service life of the photosensitive layer. Therefore, it is desirable that the photosensitive layer be resistant against wear.
On the other hand, in order to adequately transfer carriers, the CTLs 17 of the organic photosensitive layers 25 and 35 must have semiconductor characteristics. Normally, a P-type CTL is used in place of N-type CTL transfer layer. This is because the N-type CTL cannot transfer charges very fast, and also, does not unstably function. When charging the surface of the photosensitive layer, both positive and negative charge systems may be used. However, when adopting the organic photosensitive layer as the photosensitive layer, since the N-type CTL cannot properly function itself as mentioned above, when introducing the positive charge system, an available photosensitive layer is solely composed of the CGL/CTL/substrate structure . On the other hand, when introducing the negative charge system, the other composition of the CTL/CGL/substrate structure is solely used.
Recently, a semiconductor laser of AlGaAs has become widely available as a light source employed in a laser printer. This is because the semiconductor laser is small in size and can simplify an optical system, thereby realizing a significant reduction in size and weight, also resulting in the advantage in a reduced production costs. On the other hand, any of the conventional semiconductor lasers available today oscillates in a wavelength region of 780 nm to 850 nm in the vicinity of near infrared regions, and based on this reason, the photosensitive layer receiving laser beam must have a sharp sensitivity throughout the near infrared regions.
Technical problems in the conventional electrophotographic photosensitive body are described below.
First of all, the most critical problem is the poor resistance of the photosensitive layer against wear through repeated printing operations. When operating any conventional electrophotographic copying apparatus, since a blade comes into contact with the surface of the photosensitive layer, the photosensitive layer on surface of a photosensitive drum easily incurs damage, thus quickly degrading the copying characteristics thereof. In particular, the organic photosensitive layer easily incurs damage. Since the organic photosensitive layer has such a short service life that merely lasts at most 20,000 sheets of the copying process, the user is obliged to often replace the photosensitive drum. Especially, poor resistance against wear is the most critical problem when using a positive-charge system photosensitive drum of the CGL/CTL/substrate structure. However, the positive-charge system photosensitive drum of the CGL/CTL/substrate structure is superior to the negative charge system photosensitive drum of CTL/CGL/substrate structure in that the positive-charge system photosensitive drum stably generates charges on the surface of the organic photosensitive layer, and the negative-charge system photosensitive drum generates noise in the reproduced image as a result of infiltration of charges from the substrate into the charge generating layer. Nevertheless, as described above, it is difficult to put the positive-charge system photosensitive drum of the CGL/CTL/substrate structure into practice, since the thickness of the CGL is 0.2-0.5 .mu.m and the superficial wear and roughness deteriorate copied sheets.
Although a prior art proposed provides of a protective film made from a variety of polymers on the surface of the organic photosensitive layer in order to prevent damage from occurring, as typically disclosed in Japanese Laid-open Patent Publication No. 61-266567, for example, it has not yet yielded any convincing effect.
The next critical problem is the resistance of the organic photosensitive layer against ozonic atmosphere. Any conventional organic photosensitive layer incurs deterioration of photoelectric characteristics upon exposure to ozonic atmosphere for an extended time period. This results in lowered printing performance. To solve this problem, a system for quickly dissipating ozone from the neighborhood of the photosensitive drum has been proposed. Nevertheless, this system has not fully solved the problem. There was another idea of slightly abrading the surface of organic photosensitive layer by bringing a blade into contact with it in order to constantly remove an ozone-affected surface. However, it was quite difficult to control this abrading system in order to finely protect the surface of the organic photosensitive layer from incurring damage. Especially, in the case of the positive-charge system photosensitive drum of the CGL/CTL/substrate structure, since the thickness of the CGL is merely 0.2 to 0.5 .mu.m, the abrading system cannot easily be put to practical use.
In addition, the photosensitive layer still has a problem in its resistance to light. The electrophotographic photosensitive drum executes copying processes to alternately receive charges and light-exposure in the dark. However, when light continuously irradiates the photosensitive drum, the photosensitive characteristics thereof deteriorate. In particular, photosensitivity of the organic photosensitive layer is severely affected, and then, the light-affected photosensitive layer is no longer workable. Deterioration of the photosensitivity of the organic photosensitive layer is caused by degradation of the CTL after being irradiated by light. This in turn lowers the running performance of the carrier to cause the photosensitivity to also lower, and conversely, the residual potential rises. This consequently reduces the service life of the photosensitive drum itself. To prevent the photosensitive drum from suffering from reduced photosensitivity, for example, a prior art disclosed in the Japanese Laid-Open Patent Publication No. 57-90636 proposes a method of preventing a photosensitive layer from deteriorating in photosensitivity to light of short wavelengths. On the other hand, since a variety of electrophotographic copying apparatuses are made available for personal use today, the photosensitive layer is very frequently exposed to room light. Taking this into account, it is essential for manufacturers to properly protect the photosensitive layer from lowering in photosensitivity for light in the visible-ray regions as well. Nevertheless, actually no effective measure has yet been taken to realize this, but instead, since any of conventional electrophotographic copying apparatuses is externally shielded from light, the user must be very careful to properly handle the photosensitive drum, but actually, it cannot easily be treated.